|Publication number||US7301700 B2|
|Application number||US 11/531,138|
|Publication date||Nov 27, 2007|
|Filing date||Sep 12, 2006|
|Priority date||Sep 1, 2003|
|Also published as||EP1510838A1, EP1510838B1, US7116478, US20050078372, US20070058251|
|Publication number||11531138, 531138, US 7301700 B2, US 7301700B2, US-B2-7301700, US7301700 B2, US7301700B2|
|Inventors||Kazuhiko Momoki, Keiji Ohtaka|
|Original Assignee||Canon Kabushiki Kaisha|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Non-Patent Citations (4), Referenced by (2), Classifications (12), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation of application Ser. No. 10/930,128, filed Aug. 31, 2004 (now U.S. Pat. No. 7,116,478), which is incorporated herein in its entirety.
1. Field of the Invention
This invention relates to a polarization beam splitter used in lights of a plurality of wavelengths or bands, and relates, for example, to various optical apparatuses such as an image taking optical system, a projection optical system (projector), an image processing apparatus and a semiconductor manufacturing apparatus.
2. Description of Related Art
There is known a polarization beam splitter using dielectric material multi-layer film comprising two kinds of media. As shown in
Generally, when the refractive index of the medium on the incidence side with an interface as the boundary is defined as n1 and the refractive index of the medium on the emergence side is defined as n2, the Brewster's angle θB is given by the following expression (1):
tan θB=n2/n1 (1)
It is necessary that this relation is satisfied by a prism medium and a plurality of media forming the dielectric material. Among the refractive index np of the prism medium, the refractive index nH of a high refractive index layer forming the dielectric material and the refractive index nL of a low refractive index layer, it is necessary that the following relational expression is satisfied.
Regarding the S polarized light, reflecting film by multi-layer film interference is constituted by the use of the reflectances of the medium H of the high refractive index layer and the medium L of the low refractive index layer. It is possible to realize reflecting film for the entire area of visible light by 20 to 40 layers. Regarding the S polarized light, by increasing the number of the layers of the film, it is possible to secure an angle characteristic and a wavelength characteristic widely.
On the other hand, as is disclosed in U.S. Pat. No. 5,042,925, there is known a polarization beam splitter sandwiching an adhesive agent having birefringence, instead of dielectric material multi-layer film, between prisms. This uses the refractive index difference of a birefringent material between a normal ray and an abnormal ray, and the refractive index difference is not great, but yet by using it at a great incidence angle of about 60°, one polarized light is selectively totally reflected to thereby realize polarized light separation.
For total reflection to occur, the incidence angle need be a critical angle θC or greater, and the critical angle θC is given by the following expression:
sin θC=n2/n1 (3)
Also, as a polarization beam splitter using sub-wavelength structure (SWS) having a period of a used wavelength or less, there is known one as shown in
In a rectangular grating as shown in
At this time, irrespective of the ratio of a:b, nTE>nTM and therefore, in the direction TE, the refractive index difference between the H layer and the L layer is great, and in the direction TM, the refractive index difference between the H layer and the L layer is small. When an appropriate prism medium is adopted, the condition of the Brewster's angle is satisfied in the direction TM and the P polarized light can be transmitted. The thickness of each layer is independent on the condition of the Brewster's angle and therefore, by optimizing the film thicknesses of the H layer and the L layer, it is possible to form dielectric material multi-layer film. Thereby, the S polarized light is reflected and the function as a polarization beam splitter is obtained. This more heightens the degree of selection of a medium satisfying the condition of the Brewster's angle in P polarized light than a polarization beam splitter constituted by dielectric material thin film alone. Therefore, at the same time, it is possible to secure the reflectance in the S polarized light high. This leads to the feature that a polarization beam splitter covering the entire visible light area can be constituted by the order of 20 layers.
In the polarization beam splitter using the dielectric material multi-layer film, however, the condition of Brewster's angle is used for the transmission of P polarized light and therefore, the refractive indices of a prism glass material and the medium of the thin film are subject to the limitation of expression (2) above and also, it is difficult to secure an angle characteristic widely. This is not improved even if the number of layers is increased.
In a polarization beam splitter with a high molecular material having birefringence sandwiched between prisms, the refractive index difference of the high molecular material between a normal ray and an abnormal ray is not great and therefore, to effect total reflection, the incidence angle must be about 60° or greater, and this leads to the problem that the use of a usable optical system is limited. Also, this element uses a high molecular material or the like as a birefringent element and is therefore inferior from the viewpoints of heat resistance and light resistance.
In the polarization beam splitter of the laminated rectangular grating type using SWS, the construction is complicated and the manufacturing cost is high, and the condition of the Brewster's angle is used for the transmission of P polarized light and therefore, like the dielectric material multi-layer film, it is difficult to obtain a wide angle characteristic. Particularly, as is apparent from the structure of the grating shown in
In order to solve the above-noted problems, the polarization beam splitter of the present invention is a polarization beam splitter having a polarized beam splitting layer having structure in which a plurality of gratings parallel to a first direction are periodically disposed in a second direction orthogonal to the first direction, and is characterized in that of light incident on the polarization beam splitter, chiefly light of a polarized component parallel to the first direction is transmitted therethrough and chiefly light of a polarized component parallel to the second direction is reflected.
Here, it is desirable that for the light of the used wavelength area, the transmittance of the light of the polarized component parallel to the first direction be 90% or higher, and the reflectance of the light of the polarized component parallel to the second direction be 90% or higher. It is desirable that the used wavelength area be a visible light area. Of course, light of an ultraviolet wavelength area or light of an infrared wavelength area may be used.
Here, it is desirable that the plurality of gratings be arranged at a period shorter than the used wavelength.
Also, it is desirable that the light of the polarized component parallel to the second direction be reflected by the use of total reflection.
Also, it is desirable that when a surface of the polarization beam splitter on which a ray is incident is defined as a first surface, and this first surface and the polarized beam splitting layer are arranged so as to face each other at an angle which is not parallel, and a surface containing a normal to the first surface and a normal to the polarized beam splitting layer is defined as a second surface, the first direction be substantially parallel to the second surface.
Also, it is desirable that the polarization beam splitter have a shape of a square pole type having a diamond-shaped bottom surface obtained by two transparent members of substantially the same triangle pole shape having an isosceles triangle as a bottom surface having had their sides including the bottom sides of their respective isosceles triangles joined to each other. It is better if the first direction is disposed so as to be parallel to the diamond-shaped bottom surface.
Also, it is desirable that the material of the plurality of gratings be a dielectric material, and the space between the plurality of gratings be filled with air. Here, it is better if the dielectric material is titanium oxide (TiO2). It is desirable that when of the period at which the plurality of gratings are arranged, the rate the dielectric material occupies if f, 0.2<f<0.8 . . . (6) be satisfied. Here, it is desirable that the polarized beam splitting layer be sandwiched between two optical members. Also, it is desirable that the absolute value of the photoelasticity constant of the two optical members be smaller than 0.1×10−8 cm2/N. It is desirable that the two optical members have the same refractive index.
Here, it is better if when the refractive index of the optical members is defined as nP, and the refractive index of the dielectric material is defined nG, and of the period at which the plurality of gratings are arranged, the rate the dielectric material occupies is defined as f (filling factor), the following conditional expression is satisfied:
Also, it is better if when the refractive index of the optical members is defined as nP, and the refractive index of the dielectric material is defined as nG, and of the period at which the plurality of gratings are arranged, the rate the dielectric material occupies is defined as f (filling factor), and the incidence angle of light onto the polarization beam splitter is defined as θ, the following conditional expression is satisfied:
Also, it is better if design is made such that a ray is incident on the polarized beam splitting layer within an angle range including the Brewster's angle determined by the refractive index of the medium of the optical members and the effective refractive index of the polarized beam splitting layer for light having a polarized component parallel to the first direction.
Also, it is better if the refractive index of the medium of the optical members and the effective refractive index of the polarized beam splitting layer for the light having the polarized component parallel to the first direction are made substantially equal to each other.
It is better if the thickness d of the polarized beam splitting layer is such that the relation thereof with the used wavelength λS on the shortest wavelength side of the light of the used wavelength area satisfies the following conditional expression:
Also, it is desirable that design be made such that of the light incident on the polarization beam splitter, chiefly P polarized light is reflected, and chiefly S polarized light is transmitted.
Also, the polarization beam splitter of the present invention has a polarized beam splitting layer having structure in which a plurality of gratings parallel to a first direction are periodically disposed in a second direction orthogonal to the first direction, and is characterized in that of light incident on the polarization beam splitter, chiefly light of a polarized component parallel to the first direction is transmitted, and chiefly light of a polarized component parallel to the second direction is reflected.
Here, P polarized light and S polarized light are ordinary names, and polarized light in which an electric field vibrates in parallel to an incidence flat surface (generally a surface containing an incident ray and a normal to a boundary surface (now the surface of a polarizing element); in the present case, it is defined as a surface containing an incident ray, a reflected ray and a transmitted ray) is P polarized light, and polarized light in which an electric field vibrates in a direction orthogonal to the incidence flat surface is S polarized light.
Also, the image displaying apparatus of this application is characterized by at least one display element, an illuminating optical system for illuminating the aforementioned at least one display element with light from a light source, and a polarized beam splitting as described above.
Also, it is more desirable that the image display apparatus have a projection optical system for projecting light from the aforementioned at least one display element onto a projection surface.
Also, it is desirable that the aforementioned at least one display element be a reflection type display element. Also, it is better if the aforementioned at least one display element be a plurality of display elements, and be designed to have a color resolving system for color-resolving light from a light source for each wavelength (each color) when it is directed to the plurality of display elements, and a color combining system for combining reflected lights from the plurality of display elements. Here, it is more desirable that at least one of the color resolving system and the color combining system have the above polarization beam splitter. Further, it is better if each of the color resolving system and the color combining system has at least one of above described polarization beam splitter.
According to the present invention as previously described, regarding the polarization beam splitter, there is the effect that there can be realized a polarization beam splitter which is simple in both of structure and design, and yet has a high extinction ratio within wide ranges of wavelength characteristic and incidence angle characteristic.
Some embodiments of the present invention will hereinafter be described with reference to the drawings.
The height of the grating is 700 μm, and sufficiently satisfies conditional expression (6) for the light of the wavelength in the visible light area (light of the used wavelength area).
Conditional expression (6) is representative of a condition for completely achieving total reflection. Generally it is known that when light is incident from a medium of a high refractive index to a medium of a low refractive index, if the incidence angle thereof is a critical angle θC or greater, the light is not at all transmitted, but is totally reflected. At this time, however, light called an evanescent wave oozes in a very minute area near a boundary surface. If there is the following medium in the reach area of this light, the light will be transmitted. This phenomenon is attenuated total reflection (ATR), and a conditional expression for preventing this ATR is (6).
In the first embodiment, as per Design Example 1 in Table 1, a material having a refractive index as high as about 1.847 is selected as the glass material of the prism, and TiO2 having a refractive index as high as about 2.339 is also used as a dielectric material constituting the grating, and the filling factor of the grating is set to the range of conditional expressions (7) and (8).
Conditional expression (7) prescribes the condition for the transmission of a polarized component of A direction to the grating structure, and conditional expression (8) prescribes the condition for the total reflection of a polarized component of B direction. Here, when TE is the polarized component of A direction, and TM is the polarized component of B direction, the refractive indices in the respective directions of the structural birefringence are given by the aforementioned expressions (4) and (5).
When the refractive index of the optical member is defined as nP and the one-dimensional grating is constituted by a dielectric material nG and air (the refractive index thereof is 1), expressions (4) and (5) are expressed as (10) and (11), respectively, by the use of the filling factor f.
The Brewster's angle is given by expression (5), and a condition range set to expression (10) substituted for the right side of expression (5) is expression (2).
Consequently, expression (2) is the value of tan of the Brewster's angle θB, and prescribes the upper limit and lower limit of this value. If the upper limit and the lower limit are exceeded, that is, if the difference between the refractive indices of two media becomes great, reflectance increases and the transmission of the polarized component of A direction is hindered.
The total reflection condition is given by expression (7), and expression (11) substituted for the right side of expression (7) to thereby rearrange it is expression (3).
Consequently, expression (3) is the value of sin of the critical angle θC, and when this value is smaller than the incidence angle, total reflection occurs in the angle area of the entire light beam.
Thereby, the effective refractive indices of the gratings become substantially the same refractive indices regarding P polarized light, and cause a great refractive index difference regarding S polarized light, and the rays of the respective polarized lights are transmitted and totally reflected.
Conditional expression (6) prescribes the range of f (filling factor), and is a condition for chiefly causing structural birefringence efficiently. In the grating of SWS as shown in
If as in the above-described construction, use is made of a polarization beam splitter having the grating structure of SWS, there can be realized a reflection type liquid crystal projector which is excellent in the incidence angle characteristic and the wavelength characteristic and is therefore capable of projecting an image of very high contrast.
This application claims priority from Japanese Patent Application No. 2003-308787 filed Sep. 1, 2003, which is hereby incorporated by reference herein.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US4367921||Jul 31, 1980||Jan 11, 1983||Canon Kabushiki Kaisha||Low polarization beam splitter|
|US5042925||May 10, 1990||Aug 27, 1991||U.S. Philips Corporation||Polarization-sensitive beam splitter having a polarizing birefringent oriented polymer adhesive layer|
|US6288840||Jan 11, 2000||Sep 11, 2001||Moxtek||Imbedded wire grid polarizer for the visible spectrum|
|US6426837 *||Mar 21, 2000||Jul 30, 2002||Mems Optical, Inc.||Diffractive selectively polarizing beam splitter and beam routing prisms produced thereby|
|US6447120||May 21, 2001||Sep 10, 2002||Moxtex||Image projection system with a polarizing beam splitter|
|US6829090||Sep 30, 2002||Dec 7, 2004||Sony Corporation||Prism, projection device and optical component|
|US20050012996||Aug 16, 2004||Jan 20, 2005||Seiko Epson Corporation||Polarizer and optical device using the polarizer|
|WO2000057215A1||Mar 21, 2000||Sep 28, 2000||Mems Optical, Inc.||Diffractive selectively polarizing beam splitter and beam routing prisms produced thereby|
|1||Deer, Yi, et al. "Broadband Polarizing Beam Splitter Based on the Form Birefringence of a Subwavelenght Grating in the Quasi-Static Domain" Optics Letters Opt. Soc. America USA, vol. 29, No. 7, Apr. 1, 2004. pp. 754-756.|
|2||Honkanen M, et al. "Inverse Metal-Strip Polarizers" Applied Physics B: Laser and Optics, Springer International, Berlin, DE, vol. B68, No. 1. 1999, pp. 81-85.|
|3||Lalanne, et al. "A Transmission Polarizing Beam Splitter Grating" Journal of Optics A: Pure and Applied Optics, vol. 1, 1999, pp. 215-219.|
|4||Lopez, et al. "Wave-Plate Polarizing Beam Splitter Based on a Form-Birefringent Multilayer Grating" Optics Letters, vol. 23, No. 20, Oct. 15, 1998, pp. 1627-1629.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US9151958||Sep 2, 2010||Oct 6, 2015||Raytheon Company||Display system using a pair of polarized sources with a 3-D display mode and two 2-D display modes|
|US20070146880 *||Dec 27, 2005||Jun 28, 2007||Jvc Americas Corporation||Optical device for splitting an incident light into simultaneously spectrally separated and orthogonally polarized light beams having complementary primary color bands|
|U.S. Classification||359/489.06, 359/569, 353/20, 359/489.09, 359/489.11|
|International Classification||G03B21/00, G02B27/28, G02B5/30|
|Cooperative Classification||G02B27/283, G02B5/3025|
|European Classification||G02B5/30P, G02B27/28B|
|May 20, 2008||CC||Certificate of correction|
|Apr 27, 2011||FPAY||Fee payment|
Year of fee payment: 4
|Jul 10, 2015||REMI||Maintenance fee reminder mailed|
|Nov 27, 2015||LAPS||Lapse for failure to pay maintenance fees|
|Jan 19, 2016||FP||Expired due to failure to pay maintenance fee|
Effective date: 20151127